Fiasco highlights chasm between supplies in east and west
Water systems in the west have been described as antiquated and seriously lacking compared to other parts of Northern Ireland.
Different operating systems for water supplies exist between the east of the region and the worst affected areas to the west.
More than 10,000 properties were affected in Londonderry, Fermanagh and Tyrone last week and 9,000 earlier this week.
NI Water said sites in the west were older, with those in the east having been refurbished more recently.
This work was carried out under under Project Alpha, a 25-year Public Private Partnership contract to upgrade and operate water treatment works.
The project focuses on plants at Dunore Point, Antrim; Castor Bay, Lurgan; Moyola, Magherafelt; and Ballinrees, Coleraine.
The contract was signed with the Kelda Water Group in 2006. NI Water says a deal with the private sector was necessary because at that time the sites required huge investment which the public purse couldn't afford.
The project runs until 2031 and if a problem arises at one of those plants it is the responsibility of Kelda staff to rectify it. They are separate from NI Water staff and not involved in the industrial action.
NI Water chief Sara Venning and Department for Regional Development (DRD) Minister Danny Kennedy have consistently rejected accusations that, had the problems experienced in the west of the province affected areas such as Belfast or Lisburn, they would have been dealt with much quicker.
Ms Venning told the DRD committee: "There was no decision-making on our behalf that said rural customers are less important. Absolutely not."
Mr Kennedy had faced calls to bring in private contractors to address the issues in the west.
He told yesterday's emergency DRD committee meeting there was a reluctance among some private companies to step in due to the perception of strike-breaking.
Sinn Fein Mid Ulster MP Francie Molloy last night said it was vital investment was allocated to the west of the region.
"Public services west of the Bann are antiquated and need to be brought up to 21st century standards.
"There is no proper infrastructure, no evidence of strategic forward planning, and no proper servicing regime," he said. "The people of the west pay for the same services through their taxes yet receive poorer services. They deserve more than what they are currently getting - they deserve the same quality of services and infrastructure as other areas.
"People and businesses west of the Bann have always been aware that the roads and other infrastructure required to attract investors does not exist. But events of recent weeks have exposed just how extensive the underinvestment really is."